In the field LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains energy into the form required by the LEDs while keeping in compliance with present and future power mains regulations. In addition, it is required that the driver circuits comply with existing power adjusting means, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units.
The driver circuits should comply with all kinds of dimmers and especially the drivers should comply with phase-cut dimmers, which are preferably used to regulate the mains power with low power loss. Those dimmers which are usually used to regulate the mains energy provided to a filament lamp need a low load impedance path for a timing circuit operation current to adjust the phase-cut timing. Alternatively to providing this path continuously, making and breaking that path for certain parts of the mains voltage cycle can also result in stable operation. The provision of this low impedance path has to be adjusted with respect to the zero crossing of the mains voltage. Further, to provide a proper timing circuit operation, a high impedance state of the load has to be provided since a load current of an LED unit usually decreased rapidly after a dimmer is switched on. During this high impedance phase a leakage current of the load influences the timing circuit operation and may cause an early switching of the dimmer. In the case that the load of the dimmer consists of multiple retrofit lamp in parallel, each having an individual leakage current, the total leakage current increases accordingly and may cause an unacceptable error of the timing circuit operation, limiting the dimming range.
WO 2011/073865 A1 discloses a driver device for a solid state lamp, wherein a current detector is connected to a rectifying unit and a charge buffer device is incorporated in the driver device. The charge buffer device is provided for generating a suitable drive current and the current detector is provided for driving a current generating unit for adjusting the drive current provided to the lamp.
This driver device is provided for adjusting the drive current as desired for an LED unit, however, this driver device does not prevent an error of the timing circuit caused by a leakage current of the LED unit.